BECKWITH ELECTRIC M-4272, Instruction Book

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M-4272 Instruction Book
1–10
Residual Voltage Transfer
In a residual voltage transfer, the motor bus is
connected to the new source after the voltage on
the coasting motor bus falls to less than 0.33 pu. In
this manner, no matter what the phase angle, the
resultant V/Hz will not exceed 1.33 V/Hz. This is
the third type of transfer type that does not parallel
the sources. This type of transfer, however, typically
is not fast enough to maintain process continuity,
as certain motor loads that cause rapid stalling may
necessitate a restart of the motors on the bus.
Fixed Time Transfer
In a fixed time transfer, the motor bus is connected
to the new source after a time delay that would
reflect that the voltage on the coasting motor bus
has fallen to less than 0.33 pu. In this manner, no
matter what the phase angle, the resultant V/Hz will
not exceed 1.33 V/Hz. This is the fourth type of
transfer type that does not parallel the sources.
This type transfer, however, will not be able maintain
process continuity, as certain motor loads that
cause rapid stalling may necessitate a restart of the
motors on the bus.
These open transition transfers would be attempted
in the order they are presented, as shown in Figure
1-9. The first possible transfer would be the fast
transfer which would be made if the phase angle
between the motor bus and the new source is not
too large. If that transfer was not made, an in-
phase, residual or fixed time transfer would be
initiated. Obviously, the fast transfer offers the
greatest chance of process continuity, as the
interruption period to the motors is short. The in-
phase transfer provides one more opportunity to
transfer with a good possibilty to keep process
continuity. By the time a residual or fixed time
transfer is initiated, the motors may have stopped
against low inertia loads, such as positive
displacement pumps or conveyors, and a process
interruption may occur. In addition, if many motors
must be started to restart the process, load shedding
and staggered starting of the motors may have to
be undertaken so as not to overload the capacity of
the new source and cause unacceptable voltage
sags during the restarting procedure.
References
[1] ANSI C50.41-2000, American Nation
Standard for Polyphase Induction Motors
for Power generating Stations.
[2] A.P. Gabba and D.S. Hill, “Make
Automatic Power Source Transfers a
Success for Your Plants,”
IEEE
Transactions on Industry Applications, Vol.
37, No. 2, Mar/Apr 2001.
[3] W.G. Hartmann, “Automatic Synchronizing
for Generation and Tie Lines,” 18
th
Annual
Georgia Tech Protective MBTSing
Conference, Atlanta, GA, May 1999.
0
360
Phase (
Degrees)
Time
180
Volt
age
0 pu
1 pu
Fast
In-Phase
Residual and
Fixed Time
Figure 1-9 Order of Possible Open Transition MBTS